1. Field of the Invention
This invention relate to quality control testing of electronic circuitry and, more particularly, to equipment and methods for the non-destructive testing of integrated circuits or other microelectronic devices. This specific application uses a focused electron beam to make current/voltage measurements on photodiode arrays.
2. Description of the Related Art
The development of photodiode arrays comprising large numbers of individual elements is becoming increasingly important in certain specific applications. For quality assurance, it is virtually essential that screening of a fabricated photodiode array on an individual element basis be performed prior to incorporation of the array in utilization equipment. Heretofore, such screening has been performed by mechanical probing of the elements, usually on a single element basis. The results of such a procedure are less than satisfactory. Delicate indium bumps which are provided for circuit test contacts are easily damaged by a contacting probe. The procedure is a very time-consuming operation. A further disadvantage is that the results of the procedure are extremely operator dependent.
Given the nature of the problem--small detector size (approximately 1.5.times.1.5 mils) of typical multi-element hybrid focal plane arrays and the delicate character of the indium interconnects--some type of non-contacting method of diode probing is desired, preferably one which can be automated or at least performed in a way which provides results which are independent of operator parameters.
A system has been developed which uses an electron beam integrated circuit tester for testing the internal nodes of a complex integrated circuit. This system incorporates electron beam apparatus which focuses and directs the electron beam to selected internal nodes and detects secondary electron emission therefrom with associated computerized control circuitry. As thus controlled, the electron beam apparatus provides high speed access and testing of the integrated circuit nodes.
In one version of the system, the probe intelligently chooses a limited number of the internal nodes which are considered most likely to indicate circuit failure so as to minimize the number of nodes tested while maximizing reliability of the results. The probe comprises an artificial intelligence which understands the design and operation of the integrated circuit under test, the intelligence being embodied in a programmed computer associated with the probe. The system further includes circuitry for accessing the peripheral pads of the integrated circuit under test and for applying the proper circuit biases, clock signals and test signals under control of the computer. In such a system, the movement of the electron beam between selected internal nodes of the circuit and the operation of the associated computer to select those nodes are decisions which may be made in a matter of microseconds or milliseconds.
The electron beam of the test apparatus, when focused on a single selected node within the circuit, creates secondary electron emission having a flux which is affected by the properties of the node on which the electrons of the primary beam impinge. A suitable detector responds to the secondary emission electron flux, permitting the computer to sense and store the voltage of the node under test.
A particular advantage of such a system is that the probe does not capacitively load any of the internal nodes of the integrated circuit under test, nor does it damage them. At the same time, the electron beam of the probe has a submicrometer diameter and is easy to position with great precision. Because of the high speed with which the nodes may be selected and tested, use of the system on a production line having high product through-put is cost effective and is inherently more reliable than the conventional testing methods outlined hereinabove.
E-beam testing of individual photodiodes in a photodiode array has also been accomplished. Instead of directing the electron beam to internal circuit nodes, as described above, the beam is used to access the free electrode of a selected diode in the array. These arrays comprise individual photodiodes, one electrode of which is connected to a common ground or reference plane. The other electrodes are individually coupled to corresponding indium bumps which complete the circuit to the associated photodiodes when the diode array is mounted in utilizing apparatus. These other electrodes are also wire bonded in selected groups to an external pin connection.
In testing the photodiode array, the electron beam is deflected to the selected diode, developing a predetermined voltage thereon. The corresponding current of the diode is measured by an electrometer coupled in the circuit path to the common diode substrate reference plane. Various diode voltage levels are established by controllably varying the duty cycle of the pulsed E-beam and are measured by an associated voltage contrast sensor comprising a scintillator which responds to secondary emission electron flux from the particular diode under test. The particular I-V measurements developed in the manner described are compared with corresponding I-V plots of similar diodes of known quality characteristics, utilized as a reference, to determine if the diode under test is acceptable or not.
The reliability of the I-V measurements developed in this manner depends upon an assumption that the quality characteristics of the diode under test do not differ materially from those of the diode used as a reference. This is intended to be a reasonable assumption, since the diode used as a reference will be one which is located on the same wafer as the diode array under test, thus presumably having been fabricated under the same conditions. It does not always work out that way, however. Sometimes, for reasons which are not fully understood, there may be substantial variations in the operating parameters and characteristics of different diodes on the same wafer.
Additionally, the I-V measurements depend upon an indirect calibration of the voltage contrast sensor. The voltage levels measured by the voltage contrast sensor require resort to a look-up table, derived from an indirect calibration of the voltage contrast sensor, to determine the voltage levels of the I-V measurements taken for the diode undergoing test.
It would be preferable to calibrate the voltage contrast sensor from the actual diode under test. However, the typical low background curve for the test diode does not admit of precise determinations of the true values for different voltage levels. It would be desirable to be able to calibrate the voltage contrast sensor for an individual diode by measuring the zero-current crossings of the voltage axis with increased precision, sufficient to make the calibration reliable.
U.S. Pat. No. 4,730,158 of Kasai et al discloses apparatus using an electron beam for the testing of photodiode arrays. Samples of current are recorded as the diodes are charged at successive intervals over an RC time constant curve to develop successively increasing voltages with time. Diode voltage and current are measured at the end of each interval and the resulting data are used to develop a current-voltage characteristic for each diode. During the scanning procedure, the E-beam is stepped along rows of diodes to rapidly develop the data for all of the diodes in an array. By analysis of the I.V. curves of the respective diodes in an array, detection of defective or sub-standard diodes is facilitated. There is no direct physical contact with the photodiodes; thus the array is not affected by the test procedure. The disclosure of patent No. 4,730,158 is incorporated herein by reference.
Another patent of interest is No. 4,695,794 of Bargett et al which discloses the calibration of equipment being used for testing photodiode arrays by reference to the diode under test. The diodes are illuminated with infrared radiation and different bias voltages, developed by bombardment with an electron beam, are measured at zero current. The measured voltage values are correlated with secondary emission sensor readouts to calibrate the sensor according to the specific diode being tested. As with the previously described systems, an E-beam probe is used which permits testing of the photodiode arrays without direct physical contact of the photodiodes. The E-beam is chopped with varying duty cycles so that a succession of calibration voltage levels are derived which are used to relate the voltage contrast sensor output for a given diode with the actual voltage measurements. This calibrated output is then referenced directly in measuring diode voltage while taking corresponding current measurements for the I.V. curve data which is needed as the diode test results. The disclosure of patent No. 4,695,794 is incorporated herein by reference.